[PATCH 1/1] block: Merge bio before checking ->cached_rq

[PATCH 1/1] block: Merge bio before checking ->cached_rq

[Xiao Ni]

It checks if plug->cached_rq is empty before merging bio. But the merge action
doesn't have relationship with plug->cached_rq, it trys to merge bio with
requests within plug->mq_list. Now it checks if ->cached_rq is empty before
merging bio. If it's empty, it will miss the merge chances. So move the merge
function before checking ->cached_rq.

Signed-off-by: Xiao Ni <xni@redhat.com>
---
 block/blk-mq.c | 7 ++++---
 1 file changed, 4 insertions(+), 3 deletions(-)

diff --git a/block/blk-mq.c b/block/blk-mq.c
index 9c8dc70020bc..63bd7a34fe6f 100644
--- a/block/blk-mq.c
+++ b/block/blk-mq.c
@@ -2894,15 +2894,16 @@ static inline struct request *blk_mq_get_cached_request(struct request_queue *q,
 
 	if (!plug)
 		return NULL;
-	rq = rq_list_peek(&plug->cached_rq);
-	if (!rq || rq->q != q)
-		return NULL;
 
 	if (blk_mq_attempt_bio_merge(q, *bio, nsegs)) {
 		*bio = NULL;
 		return NULL;
 	}
 
+	rq = rq_list_peek(&plug->cached_rq);
+	if (!rq || rq->q != q)
+		return NULL;
+
 	type = blk_mq_get_hctx_type((*bio)->bi_opf);
 	hctx_type = rq->mq_hctx->type;
 	if (type != hctx_type &&
-- 
2.32.0 (Apple Git-132)

Re: [PATCH 1/1] block: Merge bio before checking ->cached_rq

[Ming Lei]

On Thu, Feb 09, 2023 at 11:19:30AM +0800, Xiao Ni wrote:
> It checks if plug->cached_rq is empty before merging bio. But the merge action
> doesn't have relationship with plug->cached_rq, it trys to merge bio with
> requests within plug->mq_list. Now it checks if ->cached_rq is empty before
> merging bio. If it's empty, it will miss the merge chances. So move the merge
> function before checking ->cached_rq.
> 
> Signed-off-by: Xiao Ni <xni@redhat.com>
> ---
>  block/blk-mq.c | 7 ++++---
>  1 file changed, 4 insertions(+), 3 deletions(-)

Looks fine,

Reviewed-by: Ming Lei <ming.lei@redhat.com>

Thanks,
Ming

Re: [PATCH 1/1] block: Merge bio before checking ->cached_rq

[Jens Axboe]

On Thu, 09 Feb 2023 11:19:30 +0800, Xiao Ni wrote:
> It checks if plug->cached_rq is empty before merging bio. But the merge action
> doesn't have relationship with plug->cached_rq, it trys to merge bio with
> requests within plug->mq_list. Now it checks if ->cached_rq is empty before
> merging bio. If it's empty, it will miss the merge chances. So move the merge
> function before checking ->cached_rq.
> 
> 
> [...]

Applied, thanks!

[1/1] block: Merge bio before checking ->cached_rq
      commit: 23f3e3272e7a4d9fb870485cd6df1e4f9539282c

Best regards,
-- 
Jens Axboe